Effects of the association on the phase behaviour of mixed solutions of hydrophobically modified and similar unmodified polymers
It is predicted theoretically and demonstrated experimentally that when solutions of hydrophobically modified associative and unmodified water soluble polymers are mixed, then over a certain range of composition the system phase separates into two distinct solutions. The modified polymers used in the experiments were the so called associative thickeners (AT) consisting of polyethylene-glycol (PEG), end-capped at both ends by alkane groups. Similar sized PEG homopolymers were employed as the non-associative chains. Overall polymer concentrations of the both phase separated solutions were measured and found to be identical. In contrast the viscosity of the two phases were several orders of magnitude different. This suggests that the polymer species in one phase are predominantly AT whereas those in the other phase are mainly of the unmodified type. The two phase region was only observed when the overall polymer concentrations were above 2% and the mole fraction of the AT below 0.65. It is shown that these results can be explained quantitatively by considering the effects of association within the Flory-Huggins scheme. In particular the critical mole fraction of AT is predicted to be 0.68, in excellent agreement with the experimental observations. The theoretical calculations are further extended to mixed solutions involving single end-capped chains and also those consisting of mixtures of hydrophobically modified and similar unmodified star shaped polymers. In the latter case, the tendency for phase separation between the two species of polymers is found to increase rapidly as the number of hydrophobes on the associating polymer molecules are made larger.
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